Apparatus and method of optical transfer and control in plasmon supporting metal nanostructures

ABSTRACT

An apparatus and method are provided for converting light into a surface plasmon polariton on a plasmon supporting nanostructure and then controlling the emission of the re-emitting light. A circuit component is also described that is constructed of a nanostructure, which is comprised of at least one plasmon supporting metal. The metal can propagate the light, as a surface plasmon polariton, through the one-dimensional nanostructure and re-emit the light. The metal is a plasmon supporting metal, for example but not limited to, gold, silver, copper, and aluminum.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related and claims priority to copending U.S.provisional patent applications: “OPTICAL TRANSFER AND DATA FLOW CONTROLIN METAL NANOSTRUCTURES VIA PLASMON PROPOGATION MODULATION” filed onNov. 2, 1999 and accorded Ser. No. 60/163,025; “NANOSCALE DEVICES FOROPTICAL COMPUTING” filed on May 19, 2000 and accorded Ser. No.60/205,958; and “NANOSCALE WAVE GUIDES AND OPTOELECTRONICS” filed onJul. 26, 2000 and accorded Ser. No. 60/220,920. All of the foregoingcopending U.S. provisional patent applications are entirely incorporatedherein by reference.

TECHNICAL FIELD

The present invention is generally related to plasmon supportingnanostructures that are used in circuit components and, moreparticularly, is related to an apparatus and method of converting lightinto surface plasmon polaritons upon a plasmon supporting metallic onedimensional nanostructure and then re-emitting another light.

BACKGROUND OF THE INVENTION

Scientists have been studying molecular electronics in an effort tocircumvent the size limitations on electronic components. This has beendone in part because molecular electronics has increased flexibility andease of processing with extremely high density information processing.Although some niche applications have been found for such materials,many problems such as robustness, processability, stability, andaddressability still exist. While smaller is very often better, suchdevices have limited bandwidth due to the capacitance of electroniccircuits. Conversely, optical information processing holds promise forsignificantly higher bandwidth devices, but suffers from even moresevere size and address ability concerns than those that limitconventional electronics. These problems result from the diffractionlimit—the spatial extent of light in a medium of refractive index n islimited by diffraction to about λ/2n, where λ is the free spacewavelength of light. Thus, although the construction of conventionalwaveguides from high index materials enables the minimum beam size to bedecreased significantly, waveguides are typically several times thisdiameter to adequately confine light via total internal reflection(TIR). Both modern lithography methods and molecular electronics havedemonstrated success at alleviating this size constraint for purelyelectronic devices, but diffraction imposes a fundamental size limit infurther shrinking devices for optical information processing.

The ability to transport optical signals through structures that aresmaller than the free-space optical wavelength relies upon one of twophysical processes taking place. One technique is for a waveguide to beconstructed of an extremely high refractive index material. Thistechnique can be accomplished in a simple fashion by using high indexglasses to form optical fibers, but even the highest indices (n is about3) shrink the limiting dimensions to λ/6, or about 100 nm for visiblelight. Since the range of angles capable of propagating in such fibersis smaller than that in a tightly focused spot, realistic visiblewaveguide dimensions of about 400 nm should be attainable. Thus, whileoptical fibers are nearly ideal for low loss, long range opticalcommunication, the size constraints imposed by diffraction limits theirincorporation into future nanoscale optical devices.

Alternatively, 30 nm diameter metallic structures have beentheoretically proposed to confine and transmit light due to the largenegative dielectric constants. Because light in such materials hasimaginary transverse wave vectors, the minimum waveguide diameter can bemade arbitrarily small. However, as such structures shrink, the metallicstructures exhibit exponentially increasing losses due to both thenegative (absorptive) and imaginary (imperfectly conductive) portions ofthe dielectric constant. Since the optical dielectric constant, ∈, isthe square of the complex refractive index, and large negativedielectric constants result from the large imaginary refractive index ofthe material, light propagating in such devices will be stronglyattenuated because of absorption, rendering them impractical for devicefabrication.

Thus, a heretofore unaddressed need exists in the industry to addressthe aforementioned deficiencies and inadequacies.

SUMMARY OF THE INVENTION

The present invention provides an apparatus and method for convertinglight into a surface plasmon polariton on a plasmon supportingnanostructure and then re-emitting light.

An exemplary embodiment of the present invention is a circuit componentconstructed of a nanostructure, which has at least one plasmonsupporting metal. The metal can propagate the light, as a surfaceplasmon polariton, through the nanostructure and re-emit the light. Thelight is propagated through the nanostructure using a one-dimensionalnanostructure-confined surface plasmon. The nanostructure can be, but isnot limited to, one-dimensional or pseudo one-dimensional (hereinafterone-dimensional). The metal is a plasmon supporting metal, e.g. gold,silver, copper, and aluminum.

The present invention can also be viewed as providing one or moremethods for converting light into a surface plasmon polariton on aplasmon supporting nanostructure and then re-emitting light. In thisregard, one such method can be broadly summarized by the followingsteps: providing a nanostructure; directing a first light into thenanostructure, wherein said first light has a first light energy;converting the first light energy into an surface plasmon polariton;transporting the surface plasmon polariton through the nanostructure;converting the surface plasmon polariton into a second light. In anotherembodiment of the present invention, a step can be added for controllingthe surface plasmon polariton propagation through the nanostructure.

In addition, the present invention can be viewed as one or moreapparatuses for converting light into a surface plasmon polariton on aplasmon supporting nanostructure and then re-emitting light. Theapparatus has a means for converting light energy into a surface plasmonpolariton. Further, the apparatus has a means for transporting thesurface plasmon polariton through the nanostructure. Furthermore, theapparatus has a means for converting the surface plasmon polariton backinto light. In an alternative embodiment, the apparatus has a means forcontrolling the surface plasmon polariton propagation through thenanostructure.

Other systems, methods, features, and advantages of the presentinvention will be or become apparent to one with skill in the art uponexamination of the following drawings and detailed description. It isintended that all such additional systems, methods, features, andadvantages be included within this description, be within the scope ofthe present invention, and be protected by the accompanying claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention can be better understood with reference to the followingdrawings. The components in the drawings are not necessarily to scale,emphasis instead being placed upon clearly illustrating the principlesof the present invention. Moreover, in the drawings, like referencenumerals designate corresponding parts throughout the several views.

FIG. 1 illustrates a nanostructure in accordance with the presentinvention.

FIGS. 2A and 2B illustrate a nanostructure similar to the oneillustrated in FIG. 1 except that the nanostructure is a bimetallicnanostucture.

FIGS. 3A and 3B illustrates two pairs of nanostructures, similar to theone illustrated in FIG. 1, that have nanoparticles placed between eachset of pairs.

FIGS. 4A and 4B illustrate a nanostructure, similar to the oneillustrated in FIG. 1, that has a biological component attached to it.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

As discussed above, the ability to transport optical signals throughstructures that are smaller than the free-space optical wavelengthrelies upon one of two physical processes taking place. The secondprocess, not discussed above, is to convert the light, photons, intoelectromagnetic modes that are confined by different spatial boundaryconditions than those imposed upon light. In other words, the opticalenergy of the light is converted into a surface plasmon polaritontransported via plasmon propagation. The current embodiments of thefollowing invention are based on this premise.

Although optical energy usually propagates through materials as light,in free electron metals (films or nanoparticles) light can beefficiently converted into surface plasmons at optical frequencies viathe metal's large negative dielectric constant. A plasmon is asurface-bound electromagnetic wave resulting from the collectiveoscillation of the free electrons within the metal. These modes aretypically excited with specific angles, wavelengths, and thereforemomenta of the incident light (photons). This effectively amounts tolight absorption by the free electron gas that excites a propagatingelectromagnetic wave within the metal. Energy propagation proceeds inthe direction of the wave vector until the plasmon mode decays fromeither collisional lattice damping, or re-emission at a scatteringcenter/discontinuity in the metal surface as an elastically scatteredphoton. Not subject to the same size and attenuation limitationsresulting from nanoscale light propagation, surface plasmons of a givenfrequency have significantly higher momenta than does light either infree space or in a medium of the same dielectric. This difference inmomentum from collective electron excitation versus light propagation iswhat confines the electromagnetic energy to the free electron metal.

When a scattering center/discontinuity in the metal film is reached theenergy couples back out to free-space as light. By utilizing the surfaceplasmon mode of a free electron metal as a conduit for optical energytransport, the conduit may be reduced to a size fundamentally limited bya combination of the Thomas-Fermi screening length (e.g. about 0.1 nm inAu) and the electron mean free path (e.g. about 4 nm in Au), just as inelectronic components. These dimensional limits are orders of magnitudesmaller than those mediating light confinement in high index materials,yet still enable both electronic and optical (plasmonic) informationtransfer, simultaneously.

A preferred embodiment of the present invention is a circuit component,as shown in FIG. 1. The circuit component 100 includes a nanostructure110 (hereinafter one-dimensional nanostructure), that upon illuminationcan convert optical energy 120 into a surface plasmon polariton whichtransfers the surface plasmon polariton down the length of thenanostucture 110 using a one-dimensional nanostructure-confined surfaceplasmon 130. After the surface plasmon polariton traverses the length ofthe nanostructure 110, the surface plasmon polariton is converted backinto optical energy 140 and emitted as a light that is characteristic ofthe incident light. The nanostructure can be, but is not limited to, aone-dimensional or pseudo one-dimensional nanostructure. The propagationbehavior is a function of the nanostructure 110 length, diameter,composition, excitation wavelength, incident polarization, and surfaceroughness. In addition, propagation altering materials or coatings suchas biological components, chemical components, or nanoparticles can beplaced on the nanostructure 110 surface to alter the emission.

Some nonlimiting examples of chemical components include, but are notlimited to, organic coatings, polymer coatings, hydrogels, conductingpolymers, alkanethiols, aryl-thiols, alkylamines, arylamines,polyimides, polystyrenes, polyacrylates, polyacrylamides, andpolyelectrolytes. Biological components can be used to analyzebiological interactions and these include, but are not limited to,protein-protein, protein-ligand, protein-DNA, DNA-DNA, protein-RNA,protein-lipid, etc. The biological components can be wild-type (ornative) structures, mutated structures, biosynthetic biomolecules,synthetically designed mimics, or any combination thereof. Additionally,the biological components may also be labeled with molecular orparticulate structures (e.g., metal nanoparticles, polymernanoparticles, semiconductor nanoparticles, fluorescent labels,radiolabels, liposomes, vesicles, glass particles, etc.) or combinationsthereof. In addition, the nanostructure 110 segments can be composed ofnon-plasmon supporting metals, semiconductors, polymers, or insulatingmaterials, provided the nanostructure is composed of enough of aplasmon-supporting metal to produce the desired results. Further detailsconcerning surface plasmon polaritons and one-dimensionalnanostructure-confined surface plasmons will be discussed below. Thecircuit component 100 is an electrical circuit, optical circuit, orcombination thereof. More specifically, the circuit component includes,but is not limited to, a unidirectional conduit, diode, transistor,amplifier, switch, filter, optical gate and combinations thereof. Thenanostructure 110 can be of various shapes, which include, but are notlimited to, nanostructures 110 with well-defined curvature,nanostructures 110 with tapered interconnects, as well as others. Thenanostructure 110 is made of at least one plasmon supporting metal. Theplasmon supporting metals include, but are not limited to, gold (Au),silver (Ag), copper(Cu), and aluminum(Al). The nanostructures 110dimensions can range in diameter from approximately 5 nm toapproximately λ/4 (about 100 nm for visible light) and in length fromapproximately 1 micron to more than 100 microns in length.

An alternative embodiment of the present invention is the method ofconverting light into a surface plasmon polariton and then converting itback into light, i.e. photons. The first step of this method involvesproviding a one-dimensional nanostructure 110 with two ends. Thenanostructure is similar to the type described above. Next, a lightsource is directed onto the first end of the nanostructure 110. Thelight source has a particular optical energy. The optical energy isconverted into a surface plasmon polariton upon the nanostructure 110surface. The surface plasmon polariton is transferred from the first endof the nanostructure 110 to the second end of the nanostructure 110using a one-dimensional surface-confined plasmon. Then the surfaceplasmon polariton is converted back into a light upon reaching thesecond end of the nanostructure 110. In addition, an alternativeembodiment allows for a step of controlling the propagation of thesurface plasmon polariton through the surface before the light isemitted. This can be done with biological components, chemicalcomponents, or nanoparticles.

A further embodiment of the present invention is an apparatus forconverting light into a surface plasmon polariton and then converting itback into light. The apparatus can have a means for providing aone-dimensional nanostructure 110 with two ends. The nanostructure 110is similar to the type described above. In addition the apparatus canhave a means for directing a light source onto the first end of thenanostructure 110. The light source or photons have a particular opticalenergy. Additionally, the apparatus has a means for converting theoptical energy into a surface plasmon polariton upon the nanostructure110 surface. Further, the apparatus has a means for transferring thesurface plasmon polariton from the first end of the nanostructure 110 tothe second end of the nanostructure 110 using a one-dimensionalsurface-confined plasmon. Furthermore, the apparatus has a means forconverting the surface plasmon polariton back into light upon reachingthe second end of the photon. In addition, an alternative embodiment ofthe apparatus has a means for controlling the propagation of the surfaceplasmon polariton through the surface before it is emitted. This can bedone with biological components, chemical components, or nanoparticles.

Plasmon propagation is not limited by light absorption resulting fromthe imaginary component of the refractive index, but is mainly limitedby the lack of perfect electron mobility. Since plasmons propagate atvelocities approaching the speed of light with extremely highbandwidths, and the dimensions governing plasmon behavior are muchsmaller than the diffraction limit, controlling plasmon propagation inreduced dimension structures promises great advances in high densityoptical information processing. Plasmon propagation over hundreds ofmicrons in nanometer-scale devices should be readily allowed, therebyenabling orders of magnitude increases in bandwidth without significantsize limitations. Propagation length limitations can be circumventedthrough plasmon amplification, a process analogous to that in opticalfiber communications. Furthermore, most of the structures describedbelow are attainable with commercial lithography techniques; thus, theeconomic barriers to the production of plasmon-based devices will besignificantly lower than those associated with many other proposedoptical computing architectures.

A surface plasmon exhibits two components, a propagating surface-boundwave parallel to the film surface and a non-propagating evanescent fielddecaying exponentially with distance perpendicular to the interface. Thesurface-bound mode has a real wavevector that is parallel to thedirection of energy transport. The evanescent field perpendicular to themetal nanostructure 110 surface, however, is, by definition, anon-propagating, loss-less mode, that with an exponential decay constantof about λ/2, extends but does not propagate into the surroundingdielectric. Interactions of this evanescent field with the materialsurrounding the nanostructure 110 is what enables detection of chemicaland biological components, as it alters plasmon transmission along thelength of the nanostructure 110.

Surface plasmon polaritons are confined to within a few nanometers ofthe surface, perpendicular to their direction of propagation. Because,upon absorbing light, thin metals, e.g., nanostructures 110, convertoptical energy into propagating surface plasmon polaritons, theplasmon-mediated transmission of electromagnetic energy is neitherlimited by the absorptive processes nor the confinement issuesrestricting light propagation through nanostructured 110 materials. Byutilizing plasmon supporting metal nanostructures 110, one-dimensionalnanostructure-confined plasmon conduits (e.g. circuit components) can beconstructed with dimensions that are orders of magnitude smaller thanthe diffraction limit, yet still readily transport electromagneticenergy at optical frequencies. Plasmon supporting metal nanostructures110 can be readily prepared utilizing plating solutions and structuresof approximately 20 nm diameter and up to approximately 40 microns inlength can be grown electrochemically. Generally, plasmon supportingmetals are used to construct the nanostructures 110. These include, butare not limited to, gold, silver, copper, aluminum, or combinations oralloys thereof. In addition, multimetal nanostructures 110 can also begrown. One nonexclusive method for performing this to simply change theplating solution during growth to produce a nanostructure 110 composedwith one or more plasmon supporting metals. These one-dimensionalnanostructures 110 can propagate surface plasmon polaritons withsub-diffraction limited dimensions.

A. The following describes a non-limiting illustrative example of apreferred embodiment of this invention that uses gold (Au) and silver(Ag) to create circuit components that can convert light into a surfaceplasmon polariton, transfer the surface plasmon polariton, convert thesurface plasmon polariton back into light by emitting light. Plasmoncoupling and propagation in both Au and Ag can be modulated by changingthe light energy. Despite the fact that both Au and Ag displayfree-electron behavior, their wavelength-dependent optical propertiesdiffer, as is evidenced by their disparate plasmon decays with distance,I(x):I(x)=exp(−2k″ _(x) x)  (Equation 1a)in which the imaginary portion of the plasmon momentum wave vector is asfollows: $\begin{matrix}{k_{x}^{''} = {\frac{\omega}{c}\left( \frac{ɛ_{1}^{\prime}ɛ_{2}}{ɛ_{1}^{\prime} + ɛ_{2}} \right)^{\frac{3}{2}}{\frac{ɛ_{1}^{''}}{2\left( ɛ_{1}^{\prime} \right)^{2}}.}}} & \text{(Equation~~~1b)}\end{matrix}$

In Equation 1b, ω is the optical excitation frequency, c is the speed oflight, ∈₁′ and ∈₂ are the real dielectric function of the metal and ofthe surrounding medium, respectively, and ∈₁″ is the imaginary part ofthe metal dielectric function. The plasmon propagation length is quitelong when either metal is illuminated at 820 nm, but is significant forAg at 532 nm. In other words, Ag behaves as a free electron metal atboth 532 nm and 820 nm, while Au behaves ideally at 820 nm. Usingequations 1, one can readily calculate that plasmon propagation lengthsin Ag are about 30 μm at 532 nm, and about 500 μm in the near infrared.

Plasmon conduits from high aspect-ratio metal nanostructures 110 havebeen made. Generally, high aspect ratio is greater than 3:1,length:width. These nanostructures 110 are solid nanoparticles,approximately 20 nm in diameter and many microns in length. Thenanostructures 110 are generally cylindrical but can also be any otherappropriate shape. The nanostructures 110 of the preferred embodimenthave been made of Au, Ag, or a combination thereof.

As shown in FIG. 1 and discussed above, when plasmon propagation isinitiated parallel to the long axis of the nanostructure 110, theincident light passes down the length of the nanostructure 110 as asurface plasmon polariton, and then re-emerge from the end as light thatis characteristic of the incident light (via plasmon scattering). Thenanostructures 110 scatter light in a manner characteristic of theirinteractions with the incident beam. Although light couples into thenanostructure 110 at all points along its long axis, coupling efficiencyshould be highest at the segment first interacting with the laser due toits high curvature. Under 532-nm illumination, the Au nanostructurescatters light from the input segment. Conversely, the Ag nanostructuredisplays strong emission at both the input and output segments. Uponillumination at 820 nm, however, both the Au and Ag nanostructuredisplay emission patterns that are similar to Ag at 532 nm; lightemanates from both the input and output segments of the nanostructure.Contrary to expectations for ordinary light scattering (where scatteringintensity should increase at shorter wavelengths), this wavelengthdependence strongly suggests plasmon propagation along the nanostructureaxis with subsequent emission at the discontinuous distal outputsegment.

Plasmon excitation can occur from any section of the nanostructure 110.Therefore, the illumination should be directed onto the input segment ofthe nanostructure 110. When the input segment of a Au nanostructure isilluminated at 532 nm, the nanostructure appears completely dark andscatters at the input segment. However, when Ag is illuminated under thesame conditions, emission is observed from the output segment. Theemission can be many microns away from the illumination point. At 820nm, however, light propagates through and out of the distal outputsegments of both nanostructures, demonstrating that both excitation ofsurface plasmon modes and re-emission at the nanostructure output are atleast partly dependent on the metal optical properties. These resultseven more clearly demonstrate that the observed behavior results fromplasmon coupling into, propagation through, and emission from thenanostructures.

B. The following is another non-limiting illustrative example of apreferred embodiment of the present invention that demonstrates ananostructure 110 diode or more generally a unidirectional conduit. Inaddition to the one-dimensional wavelength-dependent plasmon-mediatedlight propagation, Au is seen to limit the plasmon propagation lengthmore than Ag, even at 820 nm. Therefore, the wavelength and metaldependence of plasmon propagation can be used to control the directionof energy flow within a single conduit. Through construction of a Au/Agnanostructure in two discrete segments, a electro-optic heterojunction,or diode, has been constructed. As with the monolithic nanostructuresdescribed above, these segmented devices can be produced via atemplate-directed electrosynthetic technique. Generally, nanostructuresof a specific length are grown from the Au plating solution, followed byreplacement of that solution with a Ag solution. Continuedelectroplating from the new medium results in a second nanostructuresegment composed of Ag. The bimetallic nanostructure is a single entitywith a well-defined heterojunction as opposed to two separatenanoparticles in close proximity.

Since Au does not support plasmon propagation at 532 nm, plasmons willpropagate through the bimetallic nanostructure at 820 nm. FIGS. 2A and2B show a bimetallic nanostucture 210 that is composed of two segments.The first segment 212 is Au, while the second segment 214 is Ag. In FIG.2A the incident light 220 is directed at the Ag segment 214 and there isno resulting light emanating from the Au segment 212. This demonstratesthat the optical energy is not transferred down the nanostructure fromthe silver segment 214 to the Au segment 212. In contrast, lightemission is clearly observable from the Ag end of the nanostructure whenthe Au segment is illuminated at 820 nm. This demonstrates that theoptical energy of the light is converted into a surface plasmonpolariton and transferred to the silver segment 214. The plasmon modeexcited at 820 nm 220 is able to couple from the Au portion 212 into theAg portion 214 with high efficiency, and then re-emit 230 as it scattersat the end of the Ag segment 214. The plasmons propagate much farther inAg 214 than in Au 212, which enables efficient plasmon propagation inthe direction from Au 212 to Ag 214. However, in the opposite directionpropagation from Ag 214 to Au 212 sees a much steeper potential wall,allowing less light to couple through to the Au segment 212. In fact,due to the different propagation lengths in the two metals, if the samenanostructure is excited at 820 nm via the Ag segment 214, no light isemitted from the Au segment 212.

The directionality of reflection at the Ag/Au boundary is qualitativelyreproduced and understood by employing a simple two level potentialbarrier model, as shown in FIGS. 2A and 2B. In these figures, the Aupossesses a high potential 290 and the Ag has a lower but nonzeropotential 295; the light has energy slightly lower than either metallicpotential. Under these conditions, the probability decays exponentiallyin each metal, but more rapidly in Au (mimicking the actual plasmondecays). A significant directionality in the reflection and transmissioncoefficients at the interface results. As shown in FIG. 2B, when energypasses from the high potential region, Au, to the low potential region,Ag, the reflection coefficient is quite low 296. Conversely, as shown inFIG. 2A, passage from a low potential region to a high potential regionproduces a highly reflective interface 297. Thus, by controllingnanostructure composition, a directional propagation of the surfaceplasmon polariton conduit can be constructed, e.g., a simple nanoscaleopto-electronic diode.

With much higher bandwidths than similarly sized electronic devices, thecontrol of plasmons in such nanostructure unidirectional conduits orcircuit components is significant for the future development ofhigh-density optical information processing and sensing methods.Converting light into surface plasmons on plasmon supportingnanostructures demonstrates long-range, sub-diffraction lightpropagation. This is the first demonstration of long range opticalenergy transport in nanometer-scale structures via surface plasmons (orby any other method). Through control of nanostructure composition, notonly can high density optical information transfer be achieved, but alsocontrol of information transfer and therefore optical processing hasbeen demonstrated by utilizing the directional properties of a Au/Agheterojunction. Optically encoded information can be initiated and theflow controlled with nanometer-scale accuracy over distances of manymicrons.

C. The following is another illustrative nonlimiting example of apreferred embodiment of the present invention that describes atransistor, as shown in FIGS. 3A and 3B. Generally, while electricalcontrol of semiconductor doping is the current standard for transistordesign, the control of plasmon propagation lengths is usuallyaccomplished by modulation of the dielectric constant at the metalsurface. This is the premise upon which surface plasmon resonancesensing is based. Modulation of the local dielectric function changesthe resonance condition and hence the optimal wavelength or couplingangle and this approach will be used to construct nanostructuretransistors. It is insufficient in terms of device speed and robustnessto imagine controlling the dielectric through chemicaladsorption/desorption reactions at the surface. Furthermore, the speedof optical computing is compromised if electrical switching is used.Accordingly, optical switching based on bandgap excitation ofnanoparticles can be exploited to create optical gates for plasmonpropagation.

The optical properties of some semiconductor nanoparticles 310 (e.g.TiO₂, ZnO, CdS, CdSe, ZnS, ZnSe, CdTe,) can be modulated significantlywhen electrons are promoted to the conduction band. When the plasmonexcitation wavelength is resonant with an electronic transition of thesemiconductor conduction band electrons, absorptive modulation of theplasmon propagation should result, which is also operative in anear-field coupling scheme. The efficiency of light transfer, e.g., datatransfer, can be strongly modulated by the dielectric properties of theintervening medium; placement of nanoparticles 310 between two or morenanostructures will allow for modulation of through-space coupling.

An example of this is shown in FIGS. 3A and 3B. Introduction ofparticular semiconductor nanoparticles 310, for example about 5 nmdiameter TiO₂ nanoparticles, to a spatially isolated region on orbetween two or more nanostructures 320 and 325 illuminated 327, followedby band-gap excitation with a particular frequency of light 330, e.g.about 380 nm light, will produce a structure that can be switchedbetween a plasmon transmissive (380 nm off) 301 and absorptive (380 nmon) 303 state. FIG. 3A describes a structure 301 where the band-gapexcitation is off. Light 327 is directed onto a first nanostructure 320and the light 328 is re-emitted at the distal end of the firstnanostructure 320. The light 328 is not absorbed by the semiconductorparticles 310 and travels into the second nanostructure 325, whichsubsequently re-emits the light 329. However, in FIG. 3B the re-emittedlight 328 is absorbed by the activated, band-gap excitation 330,semiconductor particles 335, and no light reaches the secondnanostructure 325.

This will result in a nanostructure transistor-type 301 and 303 devicethat can be switched at rates comparable to optical switching schemes.Furthermore, the nanostructure 301 and 303 can be a wavelength specificdata filter; plasmons excited at frequencies that are not resonant withthe semiconductor nanoparticles gate will not be switched while resonantplasmons will not pass the gate. The signal modulation can be controlledby various means, such as, but not limited to, controlling the coverageof particles, the particle size, doping, and attachment chemistry(linker length).

Coupling nanostructures 110 (coupling of circuit components) will bedone to create larger and more complex circuits. The simplest case wouldinvolve the coupling of energy emitted from the tip of one nanostructure320 into the end of another nanostructure 325, as shown in FIGS. 3A and3B. Efficient coupling of the two nanostructures 320 and 325 will dependon a number of factors, including, but not limited to, tip-to-tipdistance, tip curvature, orientation of the nanostructures with respectto the propagation wavevector, nanostructure diameter and width, andnanostructure composition.

D. Nanostructure filters can be created by altering the propagationcharacteristics of the nanostructure. It is well established that alloysof two plasmon-carrying metals exhibit plasmon propagation behavior withstrikingly different wavelength dependencies. One method of altering thepropagation of the surface plasmon polariton is to dope a nanostructureconstructed of a particular metal with another metal. For example,adding small amounts of Ag to Au produces a film with significantlylower losses at short wavelengths. Conversely, the high energypropagation ability of Ag can be compromised through the addition of Au.Another method for propagation control is the use of high opticalextinction polymeric coatings or other chemical components, as discussedabove. When a high density of plasmon frequency absorbing chromophore isattached to the metal surface, specific propagating energies can befiltered out. Another method of selectively transmitting optical energyis to construct nanostructures that absorb in a narrow portion of thespectral range. In other words, introducing a spectrally broad pulseinto the nanostructure and the nanostructure would select a narrowportion of that spectrum for transmission.

E. The following is another illustrative nonlimiting example of apreferred embodiment of the present invention that describes anamplifier. Fluorescently labeling nanostructures should amplify surfaceplasmon polaritons. By optically pumping laser dyes attached to thenanostructure surfaces, broadband plasmon gain should be possible over awide range of optical and near infrared frequencies. Because fast energytransfer from the dye to the metal occurs, a small percentage of thisenergy will be observed as fluorescence, and the rest will be coupledback into the nanostructure at high angles, primarily along the lengthof the nanostructure. If the surface plasmon polariton are within thecorrect frequency range as determined by the frequencies over which dyesprovide optical gain, the surface plasmon polariton should be coherentlyamplified. Because the nanostructures necessary for the circuitcomponents are small, co-propagating lasers within the metalnanostructures are unnecessary; one may simply irradiate a large area ofthe sample in a direction perpendicular to surface plasmon polaritonpropagation to amplify signals, just as dye amplifiers are used in lasertechnology. Consequently, irradiation of a chip composed of circuitcomponents constructed with nanostructures in circuit components isneeded.

F. An additional aspect of the present invention and all of theembodiments specifically discussed is that separated nanostructures canconnect electrically. If a conductive but non-plasmonic structure isplaced between the two nanostructures, plasmon scattering and couplingwill still occur, while the ability to piggyback lower frequencyelectrical signals through the nanostructure will be maintained. Thiswould allow doing both electronic and plasmonic functions on the samenanostucture. Furthermore, the ability to do both electronic andplasmonic functions on the same nanostructure would allow performingelectronic and plasmonic computing on the same circuit component. Stillfurther both functions can be performed on a chip, where slowerelectrical switching could be used for low priority jobs and opticalmethods would be used for high bandwidth applications.

G. The following is another illustrative nonlimiting example of apreferred embodiment of the present invention that describes thedetection of biological interactions using nanostructures, as shown inFIGS. 4A and 4B. Detection of biological interactions (e.g.,protein-protein, protein-ligand, protein-DNA, DNA-DNA, protein-RNA,protein-lipid, etc.) via nanostructure surface plasmon resonance may beaccomplished by a number of possible schemes. Similar to previouspreferred embodiments, this embodiment is another example of controllingthe emission of the light. The following represent a number of preferredembodiments, although the technology is not intended to be limited assuch. In all examples, the nanostructure 420 consists of a largeaspect-ratio metal nanostructure 420 with a diameter ranging fromapproximately 10 nm to approximately 1 micron and a length ranging fromapproximately 1 micron to >100 microns. As discussed above, thenanostructures 420 can be synthesized via template-directed synthesis,lithography, subcomponent self-assembly or any other appropriate methodor combination thereof. Like previous preferred embodiments, thenanostructure 420 can be composed of one or more plasmon-supportingmetals (e.g. gold, silver, copper, aluminum) that are segregated intodiscrete segments, alloyed together, or some combination thereof. Inaddition, the nanostructure 420 segment can be composed of non-plasmonsupporting metals, semiconductors, polymers, or insulating materials,provided the nanostructure is composed of enough of a plasmon-supportingmetal to produce the desired results. The biological components 410 canbe wild-type (or native) structures, mutated structures, biosyntheticbiomolecules, synthetically designed mimics, or any combination thereof.Additionally, the biological components 410 may also be labeled withmolecular or particulate structures (e.g., metal nanoparticles, polymernanoparticles, semiconductor nanoparticles, fluorescent labels,radiolabels, liposomes, vesicles, glass particles, etc) or combinationsthereof.

More specifically, nanostructure biosensing can be accomplished byimmobilization of one biological component 410 to the nanostructuresurface 420, illumination 430 of one end of the nanostructure 420 withmonochromatic radiation, and then measuring the intensity of thatradiation emitted 440 from the distal nanostructure tip before and afterexposure to a liquid biological sample 450, as shown in FIGS. 4A and 4B.Any binding events that occur on the nanostructure 420 surface willcause a change in the local dielectric constant and hence alter theplasmon propagation length. This will be indicated by a change in theemission intensity 450 at the nanostructure 420 distal segment as shownin FIG. 4B. In addition to measuring the emission emanating from thedistal tip of the nanostructure 420, the amount of emission along thelength of the nanostructure 420 can be measured (not shown). Localincreases in dielectric constant can result in localized plasmonscattering, which will result in emission at sites along thenanostructure length other than simply at the distal tip.

H. A further nonlimiting illustrative example would be to usefluorescently labeled biological components or fluorescently labelednanostructures. The fluorescence intensity of the nanostructure duringexcitation will change as a function of the amount of immobilizedbiomolecule due to (a) plasmon intensity as a function of distance fromthe input end and position of the binding event, and/or (b) fluorescencequenching via energy transfer or electron transfer. Monitoring theintensity and/or spectral distribution of fluorescence will allow forquantitation of the amount of binding that has occurred. Thefluorescently labeled biomolecules will be excited by plasmon modesgenerated. Monitoring of the overall fluorescent intensity will providefor detection and quantitation of binding. In addition, excitation ofnanostructures with a spectrally broad source (e.g. polychromaticradiation) will result in differential transmission down the length ofthe nanostructure of the different wavelengths of light in that source.Binding events will modulate the local dielectric constant and hence thespectral characteristics of the emitted light. Binding events cantherefore be detected and quantified by measuring the spectrum and thetotal intensity emitted both from the nanostructure end and along itslength.

It should be emphasized that the above-described embodiments of thepresent invention, particularly, any “preferred” embodiments, are merelypossible examples of implementations, merely set forth for a clearunderstanding of the principles of the invention. Many variations andmodifications may be made to the above-described embodiment(s) of theinvention without departing substantially from the spirit and principlesof the invention. All such modifications and variations are intended tobe included herein within the scope of this disclosure and the presentinvention and protected by the following claims.

1. A circuit component comprising a nanostructure having at least onemetal, wherein said metal propagates a first light contacting a firstend of the nanostructure through said nanostructure and re-emits asecond light from a second end of the nanostructure that hascharacteristics corresponding to said first light, wherein saidnanostructure is coated with a propagation altering material, whereinsaid nanostructure further comprises a first segment and a secondsegment, wherein said first segment has a higher potential energybarrier and said second segment has a lower potential energy barrier,and wherein said first segment is a first plasmon supporting metal andsaid second segment is a second plasmon supporting metal.
 2. The circuitcomponent of claim 1, wherein said first light is propagated throughsaid nanostructure using a nanostructure-confined surface plasmon. 3.The circuit component of claim 1, wherein said at least one metal is aplasmon supporting metal.
 4. The circuit component of claim 1, whereinsaid at least one metal is selected from the group consisting of gold,silver, copper, and aluminum.
 5. The circuit component of claim 1,wherein said nanostructure is a unidirectional conduit.
 6. The circuitcomponent of claim 1, wherein said first plasmon supporting metal isgold.
 7. The circuit component of claim 1, wherein said second plasmonsupporting metal is silver.
 8. The circuit component of claim 1, whereinsaid nanostructure is part of a transistor.
 9. The circuit component ofclaim 1, wherein said nanostructure is part of a switch.
 10. The circuitcomponent of claim 1, wherein said nanostructure is part of anamplifier.
 11. The circuit component of claim 1, wherein saidnanostructure is part of a filter.
 12. The circuit component of claim 1,wherein said propagation altering material is a chemical component. 13.The circuit component of claim 1, wherein said propagation alteringmaterial is a biological component.
 14. The circuit component of claim13, wherein said biological component alters said reemission of thesecond light.
 15. The circuit component of claim 1, wherein saidpropagation altering material is a fluorescent label, wherein the secondlight is amplified relative to the first light, wherein the first lightis amplified by the fluorescent label as the first light propagatesthrough said nanostructure.
 16. The circuit component of claim 1,wherein said nanostructure is silver doped with gold.
 17. The circuitcomponent of claim 1, wherein said nanostructure is gold doped withsilver.
 18. The circuit component of claim 1, wherein said propagationaltering material is a plasmon frequency absorbing chromophore, whereinthe propagation energies of the first light corresponding to the plasmonfrequency absorbing chromophore are filtered out of the second light.19. The circuit component of claim 1, further comprising: a secondnanostructure having at least one metal, wherein said secondnanostructure is positioned adjacent said nanostructure, wherein saidmetal can propagate said second light from said nanostructure throughsaid second nanostructure and re-emit a third light that hascharacteristics corresponding to said first light, wherein said secondnanostructure is coated with a propagation altering material.
 20. Thecircuit component of claim 19, further comprising: a plurality ofnanoparticles disposed between said nanostructure and said secondnanostructure, wherein said nanoparticles in a first state do not absorbsaid second light, and are adapted to absorb said second light uponexcitation to a second state, wherein said nanoparticles.
 21. Thecircuit component of claim 19, further comprising: a conductive metalengaging said nanostructure and said second nanostructure, wherein saidconductive metal electronically couples an electrical signal betweensaid nanostructure and said second nanostructure.